The present invention relates generally to medical equipment and more particularly to hypodermic syringes.
As is well known, hypodermic syringes are used in the medical and veterinary fields to inject substances into humans and animals. One problem presented by the use of hypodermic syringes is protecting those working with the syringes from accidental needle-stick wounds. This is particularly important with a hypodermic syringe that has been used because, following the injection, the needle may be contaminated and spread infections such as hepatitis and HIV.
Numerous efforts have been undertaken to provide means for shielding the sharp tip of the needle of a hypodermic syringe before and after use. One category of needle shields comprises a sheath that surrounds the external surface of the needle after use. One example of a sheath type device is disclosed in U.S. Pat. No. 4,631,057 to Mitchell. The shielded needle of Mitchell comprises a syringe having an external needle guard mounted to the body of the syringe. The needle guard is placed in a retracted position to expose the needle for use, then slid forward to an extended position over the sharp end of the needle after use. U.S. Pat. No. 4,425,120 to Sampson et al. also discloses a needle guard comprising a sheath that surrounds the body of the syringe that is advanced to cover the needle after use. A common disadvantage of sheath-type safety needles is that the sheath typically covers all or substantially all of the body of the syringe and, therefore, the body of the syringe cannot be manipulated directly by the medical professional when inserting the needle. Consequently, there is substantial “play” between the surface being held by the medical professional and the needle, which makes needle insertion more difficult to control.
A second broad category of safety needles comprise the so-called “stylet-type” safety needles in which a blunting stylet or cannula is advanced through the inner lumen of the needle. U.S. Pat. No. 1,087,845 to Stevens discloses a needle in which an inner blunt cannula is extended within the inner lumen of a sharp needle by means of a thumbscrew operating a miniature rack and pinion. U.S. Pat. No. 1,527,291 to Zorraquin discloses a needle with a blunt probe that is spring loaded into its extended position. The blunt tip springs forward when the needle has passed through the wall or membrane being penetrated, thereby giving the surgeon an indication when the needle has passed through the membrane. U.S. Pat. No. 1,867,624 to Hoffman discloses a biopsy needle consisting of an inner and outer cannula. U.S. Pat. No. 2,623,521 to Shaw discloses an indicating stylet needle with a blunt probe that is spring loaded into its extended position in a manner similar to Hoffman. U.S. Pat. No. 5,009,642 to Sahi et al. discloses a syringe in which the blunting stylet is spring-loaded and snaps forward into the extended position automatically when the plunger is fully depressed. All of the above patents have relatively complex and expensive mechanisms and, in the case of the spring-loaded stylets, present the possibility of accidental deployment of the blunting stylet when trying to withdraw medicine from an ampoule. What is needed, therefore, is a safety syringe that is free from the risk of accidental self-deployment that is simple to operate and inexpensive to manufacture.